1. Field of the Invention
The present invention relates to an image forming device and particularly to an image forming device with a mechanism for adjusting pressing force between fixing members.
2. Description of the Related Art
A conventional image forming device, such as a laser printer, normally includes a process cartridge and a fixing unit. The process cartridge is for transferring toner images from a photosensitive drum onto a sheet at the time of when the sheet passes between the photosensitive drum and a transfer roller. The fixing unit is located downstream from the process cartridge in the direction that the sheet is transported, and includes a thermal roller and a pressing roller disposed in pressing contact with each other. While the sheet passes between the thermal roller and the pressing roller, heat from the thermal roller fuses the transferred toner image onto the sheet.
Sometimes a user will want to form an image on an envelope or other sheet that is thicker than a normal print sheet. Some fixing units include a switching mechanism that can reduce pressing force of the pressing roller against the thermal roller from the pressure used on a normal print sheet when such a thick sheet is to be printed on.
One such switching mechanism includes two springs for each end of the pressing roller, i.e., a total of four springs. When a thick sheet is to be printed on, then the switching mechanism switches urging force of these four springs to press the pressing roller against the thermal roller.
Another switching mechanism includes a support member that swingably supports the pressing roller and a spring that constantly urges the pressing roller to press against the thermal roller. A rod member can be freely protruded in and out to abut against the support member by an amount that depend on the thickness of the sheet to be printed on in order to switch the pressing force.